1. Field of the Invention
The present invention relates to a semiconductor laser for FM applications and, more particularly, to a hybrid semiconductor laser arrangement with both a narrow linewidth and relatively uniform FM response suitable for coherent communication and sensing applications.
2. Description of the Prior Art
Advances in the field of optical communication and sensing systems are currently directed towards a coherent system utilizing a frequency modulated laser transmitter, single mode optical fiber, and an optical heterodyne (or homodyne) receiver. Two of the most important requirements in such a coherent system are narrow laser linewidth (required for low noise mixing at the receiver) and uniform FM response (i.e., minimal changes in phase) over as broad a frequency range as possible. Narrow linewidth has been achieved with an arrangement including a conventional semiconductor laser coupled to a glass-on-silicon distributed Bragg reflector. In particular, the Bragg reflector consists of a SiO.sub.2 clad Si.sub.3 N.sub.4 core, ridge waveguide of length approximately 3 mm, with a first-order diffraction grating imposed on the top-most cladding surface. With this geometry, the Bragg reflector will exhibit a reflection band approximately 6 .ANG. wide. The laser and reflector are butt coupled, with alignment of the laser cavity to the waveguide. The resultant linewidth measurement for the hybrid device yields a linewidth (.DELTA.v) of less than 200 kHz, with a minimum of 110 kHz. A complete description of this device may be found in the article entitled "Compact Silicon-Chip Bragg Reflector Hybrid Laser with 110 kHz Linewidth", by D. A. Ackerman et al., appearing in IEEE Eleventh International Conference on Semiconductor Lasers Proceedings, Aug. 1988, at pp. 200-201.
Although the above-referenced arrangement is acceptable for certain applications, the uniformity of the FM response is also a concern, which the Ackerman et al. hybrid device does not address. In particular, there are many situations where the appearance of a phase reversal in the FM response will seriously degrade the performance of the system. For example, in an optical phase locked loop, a phase reversal in the FM response will create an unwanted error signal and thus disrupt system performance. Broadband (i.e., a few hundred MHz) and uniform FM response has been obtained utilizing, for example, a multi-electrode laser structure. The possibility of creating one such structure is discussed in the article "Small-signal response of a semiconductor laser with inhomogeneous linewidth enhancement factor: Possibilities of a flat carrier-induced FM response", by O. Nilsson et al. appearing in Applied Physics Letters, Vol. 46, No. 3, Feb. 1985, at pp. 223-5. In this article, the authors develop a series of relations which show that by introducing inhomogeneities in the linewidth enhancement factor .alpha. by creating a device including two regions with different values of the .alpha. parameter (and hence different gain values) a relatively flat FM response with no phase reversal may be obtained. However, this work does not address the issue of providing a relatively narrow linewidth.
What is lacking in the prior art, therefore, is a device which simultaneously exhibits a relatively narrow linewidth (e.g., on the order of kHz) and an essentially uniform FM response over a relatively broad range.